Electroluminescent solutions

ABSTRACT

Electroluminescent emission is produced in an active medium comprising a compound containing at least one active ion selected from the group of ions consisting of ions of cerium, praseodymium, neodymium, samarium, europium, gadolinium, dysprosium, terbium, thulium, and ytterbium. The active ion is dissolved in a solvent from which it can be electrodeposited. The preferred solvents are either a Lewis Acid in phosphorus oxychloride, dimethyl sulfoxide, or propylene carbonate. When the active medium is placed in contact with a pair of electrodes and a voltage of sufficient magnitude is applied across the electrodes, electroluminescent emission is observed.

ELECTROLUMINESCENT SOLUTIONS The invention herein described was made in the course of or under a contract or subcontract thereunder with the Department of the Navy.

BACKGROUND OF THE INVENTION The invention relates to electrical generation of luminescence and more particularly to electroluminescence in liquids.

Luninescence refers to the characteristic nonthermal emission of electromagnetic radiation by a material upon excitation. Excitation can be caused by a variety of phenomena; the type of luminescence usually being defined by the type of excitation. Electroluminescence is the production of non-thermal electromagnetic radiation by the application of an electric field; electric energy being converted directly into electromagnetic radiation without an intermediate stage.

It is well known that certain solid materials will exhibit electroluminescence. These materialshave been used to form electroluminescent devices which generally comprise a layer of electroluminescent material sandwiched between a pair of electrodes, one of which is transparent at the wavelength of light emitted by electroluminescent material. Electroluminescence will occur when the electroluminescent material is subjected to an electric field by application of a voltage across the electrodes.

Typically each solid material has a characteristic emission which occurs at a specific wavelength. New materials are required in order to generate emissions in the different regions of the electromagnetic spectrum and solid materials have not as yet been developed to produce electroluminescent emission in certain regions of the spectrum. In addition, the emission intensity is generally a linear function of the applied voltage. We have discovered that the application of an electric field in certain liquid solutions will excite the solution and produce electroluminescent emission. The region of the electromagnetic spectrum in which electroluminescent emission occurs can be controlled by changing one of the components of the solution. Electroluminescent emissions in a major portion of the electromagnetic spectrum have been achieved in this manner. In addition, the emission intensity has been found to vary as a non-linear function of applied voltage making these solutions particularly useful for flat electroluminescent displays.

SUMMARY OF THE INVENTION The invention relates to a method and apparatus for producing electroluminescent emission.

Electroluminescent emission is produced in a liquid active medium comprising a compound containing at least one active ion selected from a group of ions consisting of ions of cerium, praseodymium, neodymium, Samarium, europium, gadolinium, dysprosium, terbium, thulium, and ytterbium. This compound is dissolved in a solvent from which it can be electrodeposited, preferably either a Lewis Acid in phosphorus oxychloride, dimethyl sulfoxide or propylene carbonate. The resulting solution is placed in contact with a pair of electrodes. When a voltage of sufficient magnitude is applied across the electrodes, electroluminescence is observed.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view of an electroluminescent device utilizing the teachings of the invention.

FIG. 2 is a graph showing the dependence of peak light output on applied potential.

FIG. 3 is a graph showing the electroluminescent emission spectrum of the device utilizing terbium as the active ion.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. I there is shown an electroluminescent device comprising container 1, having active medium 2 therein. The container is constructed of a material which is non-reactive with the active medium, typically quartz or glass, amd may be of any desired size and shape. The active medium should not completely fill the container as the active medium may expand during subsequent excitation.

The active medium comprises a solute combining an active ion which is dissolved in a solvent. The solvent may be either a Lewis Acid in phosphorus oxychloride, dimethyl sulfoxide or propylene carbonate. Any of the well known Lewis Acids, such as zirconium tetrachloride (ZrCl tin tetrachloride (SnCl antimony chloride (SbCI or tin tetrabromide (SnBr may be used, however zirconium tetrachloride is preferred.

The active medium is in contact with cathode electrode 3 and anode electrode 4. In the device shown in FIG. 1, the cathode is a cylinder approximately 1 inch long with a diameter of about 40 mils and the anode is a /2-inch diameter coiled wire surrounding the cathode. The anode and cathode may also be flat plate electrodes or have any'other convenient size or shape. The anode and cathode are constructed of a suitable material, typically platinum.

A source of voltage 5 is coupled across electrodes 3 and 4 through terminals 6 and 7 respectively. Application of dc voltage initially causes the electrodeposition of the active metal onto the cathode surface. Following the deposition of the active metal onto the cathode, radiation having the characteristic emission of the active ion is observed.

It is believed that when the cathode is immersed in the solution, dynamic equilibrium is established at the cathode-solution interface with a layer of active ions surrounding the cathode. In the forward process, active metal atoms are converted to cations while in the reverse process electrons escape from the cathode. These electrons can be trapped by the cations which then become electrodeposited at the electrode surface, however, no net electrodeposition occurs.

Application of a dc potential causes a net electrodeposition of active ions on the cathode and increases the number of electrons capable of escaping from the cathode. This is essential since electroluminescence will not occur unless the cathode material in contact with the active medium is the same material as the active ion in the medium. Some of the escaping electrons acquire sufficient energy to excite the active ions near the electrode to a metastable state. Since the active medium contains only relatively heavy atoms, there are no higher energy vibrations and little or no radiationless relaxation of the ions can occur, thus enhancing the electroluminescence.

Electroluminescence has also been observed when an,

EXAMPLE I An active medium having a concentration of 0.3 M terbium ions and 0.45 M of the Lewis Acid zirconium tetrachloride in phosphorus oxychloride was prepared as follows: 32.5 grams silver oxide were dispersed in approximately 500 cc of water. 32.1 grams of trifluoroacetic acid were slowly added during which time the silver oxide dissolved completely. Twenty five grams of terbium chloride dissolved in approximately 50 cc of water were then added to the resulting solution. The precipitate, silver chloride was filtered off. The water was evaporated from the filtrate and the resulting solid terbium trifluoroacetate was collected and dried in vacuo at 50C. 10.4 grams of zirconium tetrachloride were then dissolved in 100 cc of anhydrous phosphorus oxychloride which had been purified by refluxing with lithium metal for 24 hours and distilled under nitrogen and 14.94 grams of terbium trifluoroacetate were added to the solution. Insoluble residues were removed by filtering the solution through a fine glass frit. The resulting solution was further purified by distilling off 50 percent of the volume of phosphorus oxychloride and diluting to 100 cc total volume with dry phosphorus oxychloride to produce a solution of 0.3 M terbium ions and 0.45 M zirconium tetrachloride.

The solution thus prepared was put into a cell of FIG. 1 and a dc voltage was applied across the electrodes. FIG. 2 is a graph showing the dependence of the peak light output on applied potential. Emission was first observed at about 5.5 volts dc. As shown, the emission increases slightly until a potential of about 35 volts dc is reached. As the voltage is increased above this value, the intensity of emission increases rapidly.

Application of a dc potential to the cell produces a relatively bright flash of light which lags the applied potential by approximately 2 X seconds. The flash of light lasts for about 6 X 10 seconds and then decays in about 0.2 second to a steady emission which is typically 300 times less intense than the maximum light intensity. This steady emission may further decay to half of the initial value over a period of about 10 seconds. When the voltage is removed, a faint emission persists for about 1 minute. The time that this system requires to relax in order to produce a second flash electroluminescent maximum intensity is about 4.5 seconds. The eleetroluineseent emission spectrum of this solution, shown in FIG. 3, is similar to the photoluminescent spectrum of Tb in phosphorus oxychloride.

EXAMPLES 11 THROUGH X Active media having a concentration of 0.3 M rare earth ion, RE and 0.45 M zirconium tetrachoride in phosphorus oxychloride were also prepared. The rare earth ion was selected from the group of ions consisting of ions of cerium, praseodymium, neodymium, samarium, europium, gadolinium, dysprosium, thulium and ytterbium and obtained from the rare earth oxide as follows:

The required amount of rare earth oxide as shown in Table I below was dispersed in approximately 500 cc of water and 68.4 grams of trifluoroacetic acid were added slowly until all of the oxide was dissolved.

TABLE I Amount of Rare Earth Oxide Starting Material Rare Earth Oxide Gram Amount Cerium Oxide 34.42 Prascodymium Oxide 32.98 Neodymium Oxide 33.65 Samarium Oxide 34.87 Europium Oxide 35.1) Gadolinium Oxide 36.25 Dysprosium Oxide 37.30

Thulium Oxide 38.59 Ytterbium Oxide 39.41

TABLE 11 Amount of Rare Earth Trifluoroacetate Required To Yield 0.3 M Rare Earth Rare Earth Trifluoroacetate Gram Amount Cerium 14.38 Praseodymium 14.40 Neodymium 14.50 Samarium 14.68 Europium 14.73 Gadolinium 14.89 Dysprosium 15.05 Thulium 15.24 Ytterbium 15.36

Any insoluble residues were removed by filtering through a fine glass frit. The resulting solution was purifled by distilling off 50 percent of the volume of phosphorus oxychloride and diluting with dry phosphorus oxychloride to cc total volume producing a solution of 0.3 molar rare earth ion and 0.45 molar zirconium ion.

Each of the solutions were added to a cell similar to that shown in FIG. 1. In the examples where the active ion in the solution was praseodymium, neodymium, gadolinium or thulium, the anode was in the form of a flat, platinum plate, about 1 cm wide and about 2 cm long, positioned about 1 cm from the cathode. A voltage of volts dc was applied across the electrodes of each cell and an emission which was characteristic of the active ion in the solution was observed in each.

EXAMPLE XI An active medium having a concentration of 0.3 M terbium ion in the solvent dimethyl sulfoxide was prepared byv dissolving 7.95 grams of terbium chloride to 100 cc of the solvent. The temperature was raised to about C approximately the boiling point of the solution and maintained at that temperature until all of the terbium chloride was dissolved. The active medium was placed in a cell similar to that shown in FIG. 1, however the anode was in the form of a flat, platinum plate, 1 cm wide by 2 cm long, positioned about 1 cm from the cathode. A voltage of 120 volts d.c. was applied across the electrodes and an emission which is characteristic of the terbium ion in the solution was observed.

EXAMPLE XII An active medium having a concentration of 0.3 M terbium ion in the solvent propylene carbonate was prepared by dissolving 14.94 grams terbium trifluoroacetate in 100 cc propylene carbonate. The active medium was placed in a cell similar to that shown in FIG. 1, however the anode was in the form of a flat, platinum plate, 1 cm wide by 2 cm long, positioned about l cm from the cathode. A voltage of 120 volts dc. was applied across the electrodes and an emission which is characteristic for the terbium ion in the solution was observed.

What is claimed is:

1. A method of producing electroluminescent emission comprising the steps of:

a. preparing an active medium comprising a liquid solution of:

1. a compound containing at least one active ion selected from the group of ions consisting of ions of cerium, praseodymium, neodymium, samarium, europium, gadolinium, dysprosium, terbium, thulium, and ytterbium,

2. a solvent from which active ions can be electrodeposited,

b. bringing a pair of electrodes into contact with said active medium; and

c. impressing a voltage between said electrodes, said voltage being of sufficient magnitude to produce electroluminescent emission in said active medium.

2. The method of claim 1 wherein said solvent is selected from the group of solvents consisting of a Lewis Acid in phosphorus oxychloride, dimethyl sulfoxide and propylene carbonate.

3. The method of claim 1 wherein said solvent is a Lewis Acid in phosphorus oxychloride and said active ion is terbium.

4. The method of claim 3 wherein said Lewis Acid is zirconium tetrachloride.

5. The method of claim 4 wherein said voltage is at least 5.5 volts dc.

6. An electroluminescent device comprising:

a. a container;

b. an active medium within said container, said active medium comprising a liquid solution of l. a compound containing at least one active ion selected from the group of ions consisting of ions ofcerium, praseodymium, Samarium, europium, gadolinium, terbium, dysprosium, thulium, and ytterbium,

2. a solvent from which said active ions can be electrodeposited; and

0. means for subjecting said active medium to an electric field of sufficient magnitude to produce electroluminescence.

7. The electroluminescent device of claim 6 wherein said solvent is selected from the group of solvents consisting of a Lewis Acid in phosphorus oxychloride, dimethyl sulfoxide and propylene carbonate.

8. The electroluminescent device of claim 6 wherein said means for subjecting said active medium to an electric field includes a pair of electrodes in contact with said active medium.

9. The electroluminescent device of claim 8 wherein the electrode material of one of said electrodes in contact with said active medium is the same material as the active ion in said active medium. 

2. a solvent from which active ions can be electrodeposited, b. bringing a pair of electrodes into contact with said active medium; and c. impressing a voltage between said electrodes, said voltage being of sufficient magnitude to produce electroluminescent emission in said active medium.
 2. The method of claim 1 wherein said solvent is selected from the group of solvents consisting of a Lewis Acid in phosphorus oxychloride, dimethyl sulfoxide and propylene carbonate.
 2. a solvent from which said active ions can be electrodeposited; and c. means for subjecting said active medium to an electric field of sufficient magnitude to produce electroluminescence.
 3. The method of claim 1 wherein said solvent is a Lewis Acid in phosphorus oxychloride and said active ion is terbium.
 4. The method of claim 3 wherein said Lewis Acid is zirconium tetrachloride.
 5. The method of claim 4 wherein said voltage is at least 5.5 volts dc.
 6. An electroluminescent device comprising: a. a container; b. an active medium within said container, said active medium comprising a liquid solution of
 7. The electroluminescent device of claim 6 wherein said solvent is selected from the group of solvents consisting of a Lewis Acid in phosphorus oxychloride, dimethyl sulfoxide and propylene carbonate.
 8. The electroluminescent device of claim 6 wherein said means for subjecting said active medium to an electric field includes a pair of electrodes in contact with said active medium.
 9. The electroluminescent device of claim 8 wherein the electrode material of one of said electrodes in contact with said active medium is the same material as the active ion in said active medium. 